The 3rd Generation Partnership Project (3GPP) started a release 13 Long Term Evolution (LTE) study item, Licensed Assisted Access (LAA) with at least one licensed carrier as the primary cell (PCell) and at least one unlicensed carrier as the secondary cell (SCell), aiming to use the unlicensed spectrum, on which WiFi is currently deployed. It is observed that LTE significantly impacts WiFi performance in LTE-WiFi coexistence case, if current LTE functionalities are assumed. One major reason is that WiFi follows Listen-Before-Talk (LBT) principle, which specifies that a transmitting Node can only start transmitting after it has performed Clear Channel Assessment (CCA) and measured that the channel is idle, while a legacy LTE Node does not perform CCA and may transmit continuously. An important problem for LTE Release 13 LAA is how to achieve fair and effective coexistence with Wi-Fi, and among LAA networks deployed by different operators. To ensure fair co-existence with WiFi, LTE needs to be modified to also support LBT on the unlicensed band.
There are basically two types of LBT mechanisms, Frame Based Equipment (FBE) and Load Based Equipment (LBE), where for FBE only one CCA phase is needed and the CCA is performed in pre-defined fixed timeline, thus resulting in a fixed starting time for downlink transmission, while for LBE both CCA phase and extended CCA phase are needed and the CCA may happen at any time resulting in flexible starting times for downlink transmissions.
As specified in ETSI EN 300 328, for FBE, before transmission, the equipment shall perform a Clear Channel Assessment (CCA) check. The equipment shall observe the operating channel for the duration of the CCA observation time which shall be not less than 20 μs. If the equipment finds the channel occupied, it shall not transmit on this channel during the next Fixed Frame Period. The total time during which an equipment has transmissions on a given channel without re-evaluating the availability of that channel, is defined as the Channel Occupancy Time. The Channel Occupancy Time shall be in the range 1 ms to 10 ms followed by an Idle Period of at least 5% of the Channel Occupancy Time used in the equipment for the current Fixed Frame Period.
For LBE, initial CCA is performed before using the channel and CCA observation time is minimum 20 μs. If the equipment finds the channel to be clear, it may transmit immediately. If the equipment finds the channel occupied, it shall not transmit on this channel. The equipment shall perform an Extended CCA check in which the channel is observed for the duration of a random factor R multiplied by the CCA observation time. N defines the number of clear idle slots resulting in a total Idle Period that needs to be observed before initiation of the transmission, where a slot can be interpreted as a time unit the equipment determines the medium is busy or idle. The value of N shall be randomly selected in the range 1 . . . q every time an Extended CCA is required and the value stored in a counter. The value of q is selected by the manufacturer in the range 4 . . . 32. The counter is decremented every time a CCA slot is considered to be ‘unoccupied’. When the counter reaches zero, the equipment may transmit. This Channel Occupancy Time shall be less than ( 13/32)×q ms, with q as defined above, after which the device shall perform the Extended CCA described.